What do the crochet hook sizes mean?

All crochet hook sizes are based on the diameter in millimeters, though what diameter goes with a letter can vary from company to company by a quarter millimeter or so, enough to change fabric size.

 

In fact, because sizes differ from company to company, and because U.S. and English size designations widely differ, the only sure way to identify needle size is by diameter.

 

Amongst the U.S. sizes, steel hooks range from 000 up, 000 being the largest, 00 the second largest, 0 the third, 1 the forth, and so on. Aluminum hooks usually have letter designations, "B" being the smallest and continuing through "N," then "P," "Q," and "S." Aluminum hooks can also have numbers, smallest to largest corresponding with size, which has some some difference from the letters, like a size 7 hook that does not appear in the letter designations, and size Q hook does not appear in the numbers.

 

English sizes are different. Like the difference between the American letters and numbers, English hooks have sizes American hooks do not and vice versa. They do not have any letter designations, only using numbers; the larger the number, the smaller the hook.